docs: def _swap(self, class1, class2, a):
          """evaluate cost of moving a from class1 to class2"""
          ss1 = self._ss(class1)
          ss2 = self._ss(class2)
          tss1 = ss1 + ss2
          class1c = copy.copy(class1)
          class2c = copy.copy(class2)
          class1c.remove(a)
          class2c.append(a)
          ss1 = self._ss(class1c)
          ss2 = self._ss(class2c)
          tss2 = ss1 + ss2
          if tss1 < tss2:
              return False
          else:
              return True